Re: [PATCH bpf-next 6/9] uprobes: Don't sleep on mmap_lock with dup_mmap_sem held in dup_mmap()

[sashiko-bot@kernel.org]

> From: Alexei Starovoitov <ast@kernel.org>
> 
> dup_mm() takes dup_mmap_sem for reading and then dup_mmap() sleeps on
> mmap_lock of the parent. When another thread keeps that mmap_lock busy
> register_for_each_vma() waits for the reader, and every other fork() in
> the system waits for the writer. The parent doesn't have to map the file:
> 
>   A thread 1:  mmap(MAP_POPULATE) -> mmap_read_lock(A)
>   A thread 2:  fork() -> percpu_down_read(&dup_mmap_sem),
>                mmap_write_lock_killable(A) sleeps
>   tracer:      percpu_down_write(&dup_mmap_sem) sleeps
>   others:      fork() -> percpu_down_read(&dup_mmap_sem) sleeps
> 
> dup_mmap() needs both locks. Don't sleep on one of them while holding
> the other. Sleep on mmap_lock and trylock dup_mmap_sem. If that fails sleep
> [ ... ]
>  perf_event, 4 uprobes   419 (424)   0.10 (0.44)       632   11.4
> 
> During detach: 265-293 -> 0.19-0.37.
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261022015712.1297331-1-alexei.starovoitov@gmail.com?part=6