Re: [PATCH 1/2] freezer: add unsafe versions of freezable helpers

From: Colin Cross
Date: Mon May 06 2013 - 17:55:04 EST

On Mon, May 6, 2013 at 2:43 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Mon, 6 May 2013 12:57:54 -0700
> Colin Cross <ccross@xxxxxxxxxxx> wrote:
>
>> On Mon, May 6, 2013 at 3:56 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>> > On Fri, 3 May 2013 14:04:09 -0700
>> > Colin Cross <ccross@xxxxxxxxxxx> wrote:
>> >
>> >> NFS calls the freezable helpers with locks held, which is unsafe
>> >> and caused lockdep warnings when 6aa9707 "lockdep: check that no
>> >> locks held at freeze time" was applied (reverted in dbf520a).
>> >> Add new *_unsafe versions of the helpers that will not run the
>> >> lockdep test when 6aa9707 is reapplied, and call them from NFS.
>> >>
>> >> Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
>> >> ---
>> >> fs/nfs/inode.c | 2 +-
>> >> fs/nfs/nfs3proc.c | 2 +-
>> >> fs/nfs/nfs4proc.c | 4 ++--
>> >> include/linux/freezer.h | 42 +++++++++++++++++++++++++++++++++++++++++-
>> >> net/sunrpc/sched.c | 2 +-
>> >> 5 files changed, 46 insertions(+), 6 deletions(-)
>> >>
>> >> diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
>> >> index 1f94167..53cbee5 100644
>> >> --- a/fs/nfs/inode.c
>> >> +++ b/fs/nfs/inode.c
>> >> @@ -79,7 +79,7 @@ int nfs_wait_bit_killable(void *word)
>> >> {
>> >> if (fatal_signal_pending(current))
>> >> return -ERESTARTSYS;
>> >> - freezable_schedule();
>> >> + freezable_schedule_unsafe();
>> >> return 0;
>> >> }
>> >> EXPORT_SYMBOL_GPL(nfs_wait_bit_killable);
>> >> diff --git a/fs/nfs/nfs3proc.c b/fs/nfs/nfs3proc.c
>> >> index 43ea96c..ce90eb4 100644
>> >> --- a/fs/nfs/nfs3proc.c
>> >> +++ b/fs/nfs/nfs3proc.c
>> >> @@ -33,7 +33,7 @@ nfs3_rpc_wrapper(struct rpc_clnt *clnt, struct rpc_message *msg, int flags)
>> >> res = rpc_call_sync(clnt, msg, flags);
>> >> if (res != -EJUKEBOX)
>> >> break;
>> >> - freezable_schedule_timeout_killable(NFS_JUKEBOX_RETRY_TIME);
>> >> + freezable_schedule_timeout_killable_unsafe(NFS_JUKEBOX_RETRY_TIME);
>> >> res = -ERESTARTSYS;
>> >> } while (!fatal_signal_pending(current));
>> >> return res;
>> >> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
>> >> index 0ad025e..a236077 100644
>> >> --- a/fs/nfs/nfs4proc.c
>> >> +++ b/fs/nfs/nfs4proc.c
>> >> @@ -266,7 +266,7 @@ static int nfs4_delay(struct rpc_clnt *clnt, long *timeout)
>> >> *timeout = NFS4_POLL_RETRY_MIN;
>> >> if (*timeout > NFS4_POLL_RETRY_MAX)
>> >> *timeout = NFS4_POLL_RETRY_MAX;
>> >> - freezable_schedule_timeout_killable(*timeout);
>> >> + freezable_schedule_timeout_killable_unsafe(*timeout);
>> >> if (fatal_signal_pending(current))
>> >> res = -ERESTARTSYS;
>> >> *timeout <<= 1;
>> >> @@ -4309,7 +4309,7 @@ int nfs4_proc_delegreturn(struct inode *inode, struct rpc_cred *cred, const nfs4
>> >> static unsigned long
>> >> nfs4_set_lock_task_retry(unsigned long timeout)
>> >> {
>> >> - freezable_schedule_timeout_killable(timeout);
>> >> + freezable_schedule_timeout_killable_unsafe(timeout);
>> >> timeout <<= 1;
>> >> if (timeout > NFS4_LOCK_MAXTIMEOUT)
>> >> return NFS4_LOCK_MAXTIMEOUT;
>> >> diff --git a/include/linux/freezer.h b/include/linux/freezer.h
>> >> index e70df40..5b31e21c 100644
>> >> --- a/include/linux/freezer.h
>> >> +++ b/include/linux/freezer.h
>> >> @@ -46,7 +46,11 @@ extern int freeze_kernel_threads(void);
>> >> extern void thaw_processes(void);
>> >> extern void thaw_kernel_threads(void);
>> >>
>> >> -static inline bool try_to_freeze(void)
>> >> +/*
>> >> + * DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION
>> >> + * If try_to_freeze causes a lockdep warning it means the caller may deadlock
>> >> + */
>> >> +static inline bool try_to_freeze_unsafe(void)
>> >> {
>> >> might_sleep();
>> >> if (likely(!freezing(current)))
>> >> @@ -54,6 +58,11 @@ static inline bool try_to_freeze(void)
>> >> return __refrigerator(false);
>> >> }
>> >>
>> >> +static inline bool try_to_freeze(void)
>> >> +{
>> >> + return try_to_freeze_unsafe();
>> >> +}
>> >> +
>> >> extern bool freeze_task(struct task_struct *p);
>> >> extern bool set_freezable(void);
>> >>
>> >> @@ -115,6 +124,14 @@ static inline void freezer_count(void)
>> >> try_to_freeze();
>> >> }
>> >>
>> >> +/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
>> >> +static inline void freezer_count_unsafe(void)
>> >> +{
>> >> + current->flags &= ~PF_FREEZER_SKIP;
>> >> + smp_mb();
>> >> + try_to_freeze_unsafe();
>> >> +}
>> >> +
>> >> /**
>> >> * freezer_should_skip - whether to skip a task when determining frozen
>> >> * state is reached
>> >> @@ -152,6 +169,14 @@ static inline bool freezer_should_skip(struct task_struct *p)
>> >> freezer_count(); \
>> >> })
>> >>
>> >> +/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
>> >> +#define freezable_schedule_unsafe() \
>> >> +({ \
>> >> + freezer_do_not_count(); \
>> >> + schedule(); \
>> >> + freezer_count_unsafe(); \
>> >> +})
>> >> +
>> >> /* Like schedule_timeout_killable(), but should not block the freezer. */
>> >> #define freezable_schedule_timeout_killable(timeout) \
>> >> ({ \
>> >> @@ -162,6 +187,16 @@ static inline bool freezer_should_skip(struct task_struct *p)
>> >> __retval; \
>> >> })
>> >>
>> >> +/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
>> >> +#define freezable_schedule_timeout_killable_unsafe(timeout) \
>> >> +({ \
>> >> + long __retval; \
>> >> + freezer_do_not_count(); \
>> >> + __retval = schedule_timeout_killable(timeout); \
>> >> + freezer_count_unsafe(); \
>> >> + __retval; \
>> >> +})
>> >> +
>> >> /*
>> >> * Freezer-friendly wrappers around wait_event_interruptible(),
>> >> * wait_event_killable() and wait_event_interruptible_timeout(), originally
>> >> @@ -225,9 +260,14 @@ static inline void set_freezable(void) {}
>> >>
>> >> #define freezable_schedule() schedule()
>> >>
>> >> +#define freezable_schedule_unsafe() schedule()
>> >> +
>> >> #define freezable_schedule_timeout_killable(timeout) \
>> >> schedule_timeout_killable(timeout)
>> >>
>> >> +#define freezable_schedule_timeout_killable_unsafe(timeout) \
>> >> + schedule_timeout_killable(timeout)
>> >> +
>> >> #define wait_event_freezable(wq, condition) \
>> >> wait_event_interruptible(wq, condition)
>> >>
>> >> diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
>> >> index f8529fc..8dcfadc 100644
>> >> --- a/net/sunrpc/sched.c
>> >> +++ b/net/sunrpc/sched.c
>> >> @@ -254,7 +254,7 @@ static int rpc_wait_bit_killable(void *word)
>> >> {
>> >> if (fatal_signal_pending(current))
>> >> return -ERESTARTSYS;
>> >> - freezable_schedule();
>> >> + freezable_schedule_unsafe();
>> >> return 0;
>> >> }
>> >>
>> >
>> > Looks reasonable, but note that CIFS uses wait_event_freezekillable
>> > with locks held too, which will likely have the same problem and will
>> > need the same workaround for now.
>>
>> I didn't see any lockdep warnings reported on the mailing list when
>> the lockdep patch was previously applied, can you point me to the lock
>> that is held when wait_event_freezkillable is called? I don't want to
>> add an _unsafe call where its not needed and cause more confusion.
>
> It's pretty much all of the same VFS-level locks...
>
> Basically, when a process wants to send a synchronous SMB to a CIFS
> server, it'll send off the request and then call wait_for_response() to
> wait on the reply. If you need a particular call stack, then you can
> look in the rmdir() codepath as an example:
>
> vfs_rmdir takes the i_mutex
> cifs_rmdir (via the inode ops)
> CIFSSMBRmDir (via the smb version ops)
> SendReceive
> wait_for_response
>
> ...at that point a freeze can occur while you're still holding the
> i_mutex.
>
> There are many other possibilities for other codepaths that end up in
> wait_for_response(). Once we get a solution in place for NFS, we'll
> need to do something very similar for CIFS.

Makes sense, I will add CIFS to the patch. Would you prefer it in the
same or separate patches.
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