Re: [PATCH AUTOSEL 5.5 542/542] pipe: use exclusive waits when reading or writing

From: Andrei Vagin
Date: Tue Feb 18 2020 - 04:51:22 EST

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Hi Linus,

This patch breaks one of CRIU tests. Here is a small reproducer:
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main()
{
int p[2];
pid_t p1, p2;
int status;

if (pipe(p) == -1)
return 1;

p1 = fork();
if (p1 == 0) {
close(p[1]);
read(p[0], &status, sizeof(status));
return 0;
}
p2 = fork();
if (p2 == 0) {
close(p[1]);
read(p[0], &status, sizeof(status));
return 0;
}
sleep(1);
close(p[1]);
wait(&status);
wait(&status);

return 0;
}

Here are two readers which are waiting for data but only one of them
will be woken up after closing the last writer.

The quick fix looks like this:

diff --git a/fs/pipe.c b/fs/pipe.c
index 5a34d6c22d4c..deaf67239a18 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -412,7 +412,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
wake_up_interruptible_sync_poll(&pipe->wr_wait,
EPOLLOUT | EPOLLWRNORM);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
- if (wake_next_reader)
+ if (!pipe->writers || wake_next_reader)
wake_up_interruptible_sync_poll(&pipe->rd_wait,
EPOLLIN | EPOLLRDNORM);
if (ret > 0)
file_accessed(filp);

I've checked that it fixes the issue, but It is too late today to read
this code carefully, so I could skip something.

Thanks,
Andrei

On Fri, Feb 14, 2020 at 8:03 AM Sasha Levin <sashal@xxxxxxxxxx> wrote:
>
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> [ Upstream commit 0ddad21d3e99c743a3aa473121dc5561679e26bb ]
>
> This makes the pipe code use separate wait-queues and exclusive waiting
> for readers and writers, avoiding a nasty thundering herd problem when
> there are lots of readers waiting for data on a pipe (or, less commonly,
> lots of writers waiting for a pipe to have space).
>
> While this isn't a common occurrence in the traditional "use a pipe as a
> data transport" case, where you typically only have a single reader and
> a single writer process, there is one common special case: using a pipe
> as a source of "locking tokens" rather than for data communication.
>
> In particular, the GNU make jobserver code ends up using a pipe as a way
> to limit parallelism, where each job consumes a token by reading a byte
> from the jobserver pipe, and releases the token by writing a byte back
> to the pipe.
>
> This pattern is fairly traditional on Unix, and works very well, but
> will waste a lot of time waking up a lot of processes when only a single
> reader needs to be woken up when a writer releases a new token.
>
> A simplified test-case of just this pipe interaction is to create 64
> processes, and then pass a single token around between them (this
> test-case also intentionally passes another token that gets ignored to
> test the "wake up next" logic too, in case anybody wonders about it):
>
> #include <unistd.h>
>
> int main(int argc, char **argv)
> {
> int fd[2], counters[2];
>
> pipe(fd);
> counters[0] = 0;
> counters[1] = -1;
> write(fd[1], counters, sizeof(counters));
>
> /* 64 processes */
> fork(); fork(); fork(); fork(); fork(); fork();
>
> do {
> int i;
> read(fd[0], &i, sizeof(i));
> if (i < 0)
> continue;
> counters[0] = i+1;
> write(fd[1], counters, (1+(i & 1)) *sizeof(int));
> } while (counters[0] < 1000000);
> return 0;
> }
>
> and in a perfect world, passing that token around should only cause one
> context switch per transfer, when the writer of a token causes a
> directed wakeup of just a single reader.
>
> But with the "writer wakes all readers" model we traditionally had, on
> my test box the above case causes more than an order of magnitude more
> scheduling: instead of the expected ~1M context switches, "perf stat"
> shows
>
> 231,852.37 msec task-clock # 15.857 CPUs utilized
> 11,250,961 context-switches # 0.049 M/sec
> 616,304 cpu-migrations # 0.003 M/sec
> 1,648 page-faults # 0.007 K/sec
> 1,097,903,998,514 cycles # 4.735 GHz
> 120,781,778,352 instructions # 0.11 insn per cycle
> 27,997,056,043 branches # 120.754 M/sec
> 283,581,233 branch-misses # 1.01% of all branches
>
> 14.621273891 seconds time elapsed
>
> 0.018243000 seconds user
> 3.611468000 seconds sys
>
> before this commit.
>
> After this commit, I get
>
> 5,229.55 msec task-clock # 3.072 CPUs utilized
> 1,212,233 context-switches # 0.232 M/sec
> 103,951 cpu-migrations # 0.020 M/sec
> 1,328 page-faults # 0.254 K/sec
> 21,307,456,166 cycles # 4.074 GHz
> 12,947,819,999 instructions # 0.61 insn per cycle
> 2,881,985,678 branches # 551.096 M/sec
> 64,267,015 branch-misses # 2.23% of all branches
>
> 1.702148350 seconds time elapsed
>
> 0.004868000 seconds user
> 0.110786000 seconds sys
>
> instead. Much better.
>
> [ Note! This kernel improvement seems to be very good at triggering a
> race condition in the make jobserver (in GNU make 4.2.1) for me. It's
> a long known bug that was fixed back in June 2017 by GNU make commit
> b552b0525198 ("[SV 51159] Use a non-blocking read with pselect to
> avoid hangs.").
>
> But there wasn't a new release of GNU make until 4.3 on Jan 19 2020,
> so a number of distributions may still have the buggy version. Some
> have backported the fix to their 4.2.1 release, though, and even
> without the fix it's quite timing-dependent whether the bug actually
> is hit. ]
>
> Josh Triplett says:
> "I've been hammering on your pipe fix patch (switching to exclusive
> wait queues) for a month or so, on several different systems, and I've
> run into no issues with it. The patch *substantially* improves
> parallel build times on large (~100 CPU) systems, both with parallel
> make and with other things that use make's pipe-based jobserver.
>
> All current distributions (including stable and long-term stable
> distributions) have versions of GNU make that no longer have the
> jobserver bug"
>
> Tested-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
> fs/coredump.c | 4 +--
> fs/pipe.c | 67 +++++++++++++++++++++++++--------------
> fs/splice.c | 8 ++---
> include/linux/pipe_fs_i.h | 2 +-
> 4 files changed, 51 insertions(+), 30 deletions(-)
>
> diff --git a/fs/coredump.c b/fs/coredump.c
> index b1ea7dfbd1494..f8296a82d01df 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -517,7 +517,7 @@ static void wait_for_dump_helpers(struct file *file)
> pipe_lock(pipe);
> pipe->readers++;
> pipe->writers--;
> - wake_up_interruptible_sync(&pipe->wait);
> + wake_up_interruptible_sync(&pipe->rd_wait);
> kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> pipe_unlock(pipe);
>
> @@ -525,7 +525,7 @@ static void wait_for_dump_helpers(struct file *file)
> * We actually want wait_event_freezable() but then we need
> * to clear TIF_SIGPENDING and improve dump_interrupted().
> */
> - wait_event_interruptible(pipe->wait, pipe->readers == 1);
> + wait_event_interruptible(pipe->rd_wait, pipe->readers == 1);
>
> pipe_lock(pipe);
> pipe->readers--;
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 57502c3c0fba1..5a34d6c22d4ce 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -108,16 +108,19 @@ void pipe_double_lock(struct pipe_inode_info *pipe1,
> /* Drop the inode semaphore and wait for a pipe event, atomically */
> void pipe_wait(struct pipe_inode_info *pipe)
> {
> - DEFINE_WAIT(wait);
> + DEFINE_WAIT(rdwait);
> + DEFINE_WAIT(wrwait);
>
> /*
> * Pipes are system-local resources, so sleeping on them
> * is considered a noninteractive wait:
> */
> - prepare_to_wait(&pipe->wait, &wait, TASK_INTERRUPTIBLE);
> + prepare_to_wait(&pipe->rd_wait, &rdwait, TASK_INTERRUPTIBLE);
> + prepare_to_wait(&pipe->wr_wait, &wrwait, TASK_INTERRUPTIBLE);
> pipe_unlock(pipe);
> schedule();
> - finish_wait(&pipe->wait, &wait);
> + finish_wait(&pipe->rd_wait, &rdwait);
> + finish_wait(&pipe->wr_wait, &wrwait);
> pipe_lock(pipe);
> }
>
> @@ -286,7 +289,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
> size_t total_len = iov_iter_count(to);
> struct file *filp = iocb->ki_filp;
> struct pipe_inode_info *pipe = filp->private_data;
> - bool was_full;
> + bool was_full, wake_next_reader = false;
> ssize_t ret;
>
> /* Null read succeeds. */
> @@ -344,10 +347,10 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
>
> if (!buf->len) {
> pipe_buf_release(pipe, buf);
> - spin_lock_irq(&pipe->wait.lock);
> + spin_lock_irq(&pipe->rd_wait.lock);
> tail++;
> pipe->tail = tail;
> - spin_unlock_irq(&pipe->wait.lock);
> + spin_unlock_irq(&pipe->rd_wait.lock);
> }
> total_len -= chars;
> if (!total_len)
> @@ -384,7 +387,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
> * no data.
> */
> if (unlikely(was_full)) {
> - wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
> + wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
> kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
> }
>
> @@ -394,18 +397,23 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
> * since we've done any required wakeups and there's no need
> * to mark anything accessed. And we've dropped the lock.
> */
> - if (wait_event_interruptible(pipe->wait, pipe_readable(pipe)) < 0)
> + if (wait_event_interruptible_exclusive(pipe->rd_wait, pipe_readable(pipe)) < 0)
> return -ERESTARTSYS;
>
> __pipe_lock(pipe);
> was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
> + wake_next_reader = true;
> }
> + if (pipe_empty(pipe->head, pipe->tail))
> + wake_next_reader = false;
> __pipe_unlock(pipe);
>
> if (was_full) {
> - wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
> + wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
> kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
> }
> + if (wake_next_reader)
> + wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
> if (ret > 0)
> file_accessed(filp);
> return ret;
> @@ -437,6 +445,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> size_t total_len = iov_iter_count(from);
> ssize_t chars;
> bool was_empty = false;
> + bool wake_next_writer = false;
>
> /* Null write succeeds. */
> if (unlikely(total_len == 0))
> @@ -515,16 +524,16 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> * it, either the reader will consume it or it'll still
> * be there for the next write.
> */
> - spin_lock_irq(&pipe->wait.lock);
> + spin_lock_irq(&pipe->rd_wait.lock);
>
> head = pipe->head;
> if (pipe_full(head, pipe->tail, pipe->max_usage)) {
> - spin_unlock_irq(&pipe->wait.lock);
> + spin_unlock_irq(&pipe->rd_wait.lock);
> continue;
> }
>
> pipe->head = head + 1;
> - spin_unlock_irq(&pipe->wait.lock);
> + spin_unlock_irq(&pipe->rd_wait.lock);
>
> /* Insert it into the buffer array */
> buf = &pipe->bufs[head & mask];
> @@ -576,14 +585,17 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> */
> __pipe_unlock(pipe);
> if (was_empty) {
> - wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLRDNORM);
> + wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
> kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> }
> - wait_event_interruptible(pipe->wait, pipe_writable(pipe));
> + wait_event_interruptible_exclusive(pipe->wr_wait, pipe_writable(pipe));
> __pipe_lock(pipe);
> was_empty = pipe_empty(pipe->head, pipe->tail);
> + wake_next_writer = true;
> }
> out:
> + if (pipe_full(pipe->head, pipe->tail, pipe->max_usage))
> + wake_next_writer = false;
> __pipe_unlock(pipe);
>
> /*
> @@ -596,9 +608,11 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> * wake up pending jobs
> */
> if (was_empty) {
> - wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLRDNORM);
> + wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
> kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> }
> + if (wake_next_writer)
> + wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
> if (ret > 0 && sb_start_write_trylock(file_inode(filp)->i_sb)) {
> int err = file_update_time(filp);
> if (err)
> @@ -642,12 +656,15 @@ pipe_poll(struct file *filp, poll_table *wait)
> unsigned int head, tail;
>
> /*
> - * Reading only -- no need for acquiring the semaphore.
> + * Reading pipe state only -- no need for acquiring the semaphore.
> *
> * But because this is racy, the code has to add the
> * entry to the poll table _first_ ..
> */
> - poll_wait(filp, &pipe->wait, wait);
> + if (filp->f_mode & FMODE_READ)
> + poll_wait(filp, &pipe->rd_wait, wait);
> + if (filp->f_mode & FMODE_WRITE)
> + poll_wait(filp, &pipe->wr_wait, wait);
>
> /*
> * .. and only then can you do the racy tests. That way,
> @@ -706,7 +723,8 @@ pipe_release(struct inode *inode, struct file *file)
> pipe->writers--;
>
> if (pipe->readers || pipe->writers) {
> - wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLOUT | EPOLLRDNORM | EPOLLWRNORM | EPOLLERR | EPOLLHUP);
> + wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM | EPOLLERR | EPOLLHUP);
> + wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM | EPOLLERR | EPOLLHUP);
> kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
> }
> @@ -789,7 +807,8 @@ struct pipe_inode_info *alloc_pipe_info(void)
> GFP_KERNEL_ACCOUNT);
>
> if (pipe->bufs) {
> - init_waitqueue_head(&pipe->wait);
> + init_waitqueue_head(&pipe->rd_wait);
> + init_waitqueue_head(&pipe->wr_wait);
> pipe->r_counter = pipe->w_counter = 1;
> pipe->max_usage = pipe_bufs;
> pipe->ring_size = pipe_bufs;
> @@ -1007,7 +1026,8 @@ static int wait_for_partner(struct pipe_inode_info *pipe, unsigned int *cnt)
>
> static void wake_up_partner(struct pipe_inode_info *pipe)
> {
> - wake_up_interruptible(&pipe->wait);
> + wake_up_interruptible(&pipe->rd_wait);
> + wake_up_interruptible(&pipe->wr_wait);
> }
>
> static int fifo_open(struct inode *inode, struct file *filp)
> @@ -1118,13 +1138,13 @@ static int fifo_open(struct inode *inode, struct file *filp)
>
> err_rd:
> if (!--pipe->readers)
> - wake_up_interruptible(&pipe->wait);
> + wake_up_interruptible(&pipe->wr_wait);
> ret = -ERESTARTSYS;
> goto err;
>
> err_wr:
> if (!--pipe->writers)
> - wake_up_interruptible(&pipe->wait);
> + wake_up_interruptible(&pipe->rd_wait);
> ret = -ERESTARTSYS;
> goto err;
>
> @@ -1251,7 +1271,8 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
> pipe->max_usage = nr_slots;
> pipe->tail = tail;
> pipe->head = head;
> - wake_up_interruptible_all(&pipe->wait);
> + wake_up_interruptible_all(&pipe->rd_wait);
> + wake_up_interruptible_all(&pipe->wr_wait);
> return pipe->max_usage * PAGE_SIZE;
>
> out_revert_acct:
> diff --git a/fs/splice.c b/fs/splice.c
> index 3009652a41c85..d671936d0aad6 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -165,8 +165,8 @@ static const struct pipe_buf_operations user_page_pipe_buf_ops = {
> static void wakeup_pipe_readers(struct pipe_inode_info *pipe)
> {
> smp_mb();
> - if (waitqueue_active(&pipe->wait))
> - wake_up_interruptible(&pipe->wait);
> + if (waitqueue_active(&pipe->rd_wait))
> + wake_up_interruptible(&pipe->rd_wait);
> kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> }
>
> @@ -462,8 +462,8 @@ static int pipe_to_sendpage(struct pipe_inode_info *pipe,
> static void wakeup_pipe_writers(struct pipe_inode_info *pipe)
> {
> smp_mb();
> - if (waitqueue_active(&pipe->wait))
> - wake_up_interruptible(&pipe->wait);
> + if (waitqueue_active(&pipe->wr_wait))
> + wake_up_interruptible(&pipe->wr_wait);
> kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
> }
>
> diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
> index dbcfa68923842..d5765039652a5 100644
> --- a/include/linux/pipe_fs_i.h
> +++ b/include/linux/pipe_fs_i.h
> @@ -47,7 +47,7 @@ struct pipe_buffer {
> **/
> struct pipe_inode_info {
> struct mutex mutex;
> - wait_queue_head_t wait;
> + wait_queue_head_t rd_wait, wr_wait;
> unsigned int head;
> unsigned int tail;
> unsigned int max_usage;
> --
> 2.20.1
>

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