Re: [PATCH] kexec: change locking mechanism to a mutex

[Dave Young]

[Cced Valentin Schneider as he added the trylocks]

On Fri, 22 Sept 2023 at 06:04, Eric DeVolder <eric.devolder@xxxxxxxxxx> wrote:
>
> Scaled up testing has revealed that the kexec_trylock()
> implementation leads to failures within the crash hotplug
> infrastructure due to the inability to acquire the lock,
> specifically the message:
>
>  crash hp: kexec_trylock() failed, elfcorehdr may be inaccurate
>
> When hotplug events occur, the crash hotplug infrastructure first
> attempts to obtain the lock via the kexec_trylock(). However, the
> implementation either acquires the lock, or fails and returns; there
> is no waiting on the lock. Here is the comment/explanation from
> kernel/kexec_internal.h:kexec_trylock():
>
>  * Whatever is used to serialize accesses to the kexec_crash_image needs to be
>  * NMI safe, as __crash_kexec() can happen during nmi_panic(), so here we use a
>  * "simple" atomic variable that is acquired with a cmpxchg().
>
> While this in theory can happen for either CPU or memory hoptlug,
> this problem is most prone to occur for memory hotplug.
>
> When memory is hot plugged, the memory is converted into smaller
> 128MiB memblocks (typically). As each memblock is processed, a
> kernel thread and a udev event thread are created. The udev thread
> tries for the lock via the reading of the sysfs node
> /sys/devices/system/memory/crash_hotplug node, and the kernel
> worker thread tries for the lock upon entering the crash hotplug
> infrastructure.
>
> These threads then compete for the kexec lock.
>
> For example, a 1GiB DIMM is converted into 8 memblocks, each
> spawning two threads for a total of 16 threads that create a small
> "swarm" all trying to acquire the lock. The larger the DIMM, the
> more the memblocks and the larger the swarm.
>
> At the root of the problem is the atomic lock behind kexec_trylock();
> it works well for low lock traffic; ie loading/unloading a capture
> kernel, things that happen basically once. But with the introduction
> of crash hotplug, the traffic through the lock increases significantly,
> and more importantly in bursts occurring at roughly the same time. Thus
> there is a need to wait on the lock.
>
> A possible workaround is to simply retry the lock, say up to N times.
> There is, of course, the problem of determining a value of N that works for
> all implementations, and for all the other call sites of kexec_trylock().
> Not ideal.
>
> The design decision to use the atomic lock is described in the comment
> from kexec_internal.h, cited above. However, examining the code of
> __crash_kexec():
>
>         if (kexec_trylock()) {
>                 if (kexec_crash_image) {
>                         ...
>                 }
>                 kexec_unlock();
>         }
>
> reveals that the use of kexec_trylock() here is actually a "best effort"
> due to the atomic lock.  This atomic lock, prior to crash hotplug,
> would almost always be assured (another kexec syscall could hold the lock
> and prevent this, but that is about it).
>
> So at the point where the capture kernel would be invoked, if the lock
> is not obtained, then kdump doesn't occur.
>
> It is possible to instead use a mutex with proper waiting, and utilize
> mutex_trylock() as the "best effort" in __crash_kexec(). The use of a
> mutex then avoids all the lock acquisition problems that were revealed
> by the crash hotplug activity.
>
> Convert the atomic lock to a mutex.
>
> Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> ---
>  kernel/crash_core.c     | 10 ++--------
>  kernel/kexec.c          |  3 +--
>  kernel/kexec_core.c     | 13 +++++--------
>  kernel/kexec_file.c     |  3 +--
>  kernel/kexec_internal.h | 12 +++---------
>  5 files changed, 12 insertions(+), 29 deletions(-)
>
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 03a7932cde0a..9a8378fbdafa 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -749,10 +749,7 @@ int crash_check_update_elfcorehdr(void)
>         int rc = 0;
>
>         /* Obtain lock while reading crash information */
> -       if (!kexec_trylock()) {
> -               pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
> -               return 0;
> -       }
> +       kexec_lock();
>         if (kexec_crash_image) {
>                 if (kexec_crash_image->file_mode)
>                         rc = 1;
> @@ -784,10 +781,7 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
>         struct kimage *image;
>
>         /* Obtain lock while changing crash information */
> -       if (!kexec_trylock()) {
> -               pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
> -               return;
> -       }
> +       kexec_lock();
>
>         /* Check kdump is not loaded */
>         if (!kexec_crash_image)
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index 107f355eac10..a2f687900bb5 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -96,8 +96,7 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
>          * crash kernels we need a serialization here to prevent multiple crash
>          * kernels from attempting to load simultaneously.
>          */
> -       if (!kexec_trylock())
> -               return -EBUSY;
> +       kexec_lock();
>
>         if (flags & KEXEC_ON_CRASH) {
>                 dest_image = &kexec_crash_image;
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 9dc728982d79..202e4590fc1c 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -47,7 +47,7 @@
>  #include <crypto/hash.h>
>  #include "kexec_internal.h"
>
> -atomic_t __kexec_lock = ATOMIC_INIT(0);
> +DEFINE_MUTEX(__kexec_lock);
>
>  /* Flag to indicate we are going to kexec a new kernel */
>  bool kexec_in_progress = false;
> @@ -1057,7 +1057,7 @@ void __noclone __crash_kexec(struct pt_regs *regs)
>          * of memory the xchg(&kexec_crash_image) would be
>          * sufficient.  But since I reuse the memory...
>          */
> -       if (kexec_trylock()) {
> +       if (mutex_trylock(&__kexec_lock)) {
>                 if (kexec_crash_image) {
>                         struct pt_regs fixed_regs;
>
> @@ -1103,8 +1103,7 @@ ssize_t crash_get_memory_size(void)
>  {
>         ssize_t size = 0;
>
> -       if (!kexec_trylock())
> -               return -EBUSY;
> +       kexec_lock();
>
>         size += crash_resource_size(&crashk_res);
>         size += crash_resource_size(&crashk_low_res);
> @@ -1146,8 +1145,7 @@ int crash_shrink_memory(unsigned long new_size)
>         int ret = 0;
>         unsigned long old_size, low_size;
>
> -       if (!kexec_trylock())
> -               return -EBUSY;
> +       kexec_lock();
>
>         if (kexec_crash_image) {
>                 ret = -ENOENT;
> @@ -1229,8 +1227,7 @@ int kernel_kexec(void)
>  {
>         int error = 0;
>
> -       if (!kexec_trylock())
> -               return -EBUSY;
> +       kexec_lock();
>         if (!kexec_image) {
>                 error = -EINVAL;
>                 goto Unlock;
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index f9a419cd22d4..a4daaaab7fa7 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -341,8 +341,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
>
>         image = NULL;
>
> -       if (!kexec_trylock())
> -               return -EBUSY;
> +       kexec_lock();
>
>         if (image_type == KEXEC_TYPE_CRASH) {
>                 dest_image = &kexec_crash_image;
> diff --git a/kernel/kexec_internal.h b/kernel/kexec_internal.h
> index 74da1409cd14..4fdae59767b6 100644
> --- a/kernel/kexec_internal.h
> +++ b/kernel/kexec_internal.h
> @@ -18,15 +18,9 @@ int kimage_is_destination_range(struct kimage *image,
>   * NMI safe, as __crash_kexec() can happen during nmi_panic(), so here we use a
>   * "simple" atomic variable that is acquired with a cmpxchg().
>   */
> -extern atomic_t __kexec_lock;
> -static inline bool kexec_trylock(void)
> -{
> -       return atomic_cmpxchg_acquire(&__kexec_lock, 0, 1) == 0;
> -}
> -static inline void kexec_unlock(void)
> -{
> -       atomic_set_release(&__kexec_lock, 0);
> -}
> +extern struct mutex __kexec_lock;
> +#define kexec_lock() mutex_lock(&__kexec_lock)
> +#define kexec_unlock() mutex_unlock(&__kexec_lock)
>
>  #ifdef CONFIG_KEXEC_FILE
>  #include <linux/purgatory.h>
> --
> 2.39.3
>