* Suren Baghdasaryan <surenb@xxxxxxxxxx> [241121 11:28]:
> Add helper functions to speculatively perform operations without
> read-locking mmap_lock, expecting that mmap_lock will not be
> write-locked and mm is not modified from under us.
>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
> Changes since v1 [1]
> - Changed to use new raw_seqcount_try_begin() API, per Peter Zijlstra
> - Renamed mmap_lock_speculation_{begin|end} into
> mmap_lock_speculate_{try_begin|retry}, per Peter Zijlstra
>
> Note: the return value of mmap_lock_speculate_retry() is opposive to
> what it was in mmap_lock_speculation_end(). true now means speculation failed.
>
> [1] https://lore.kernel.org/all/20241024205231.1944747-2-surenb@xxxxxxxxxx/
>
> include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
> 1 file changed, 31 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 083b7fa2588e..0b39a0f99a3b 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> }
>
> #ifdef CONFIG_PER_VMA_LOCK
> +
> static inline void mm_lock_seqcount_init(struct mm_struct *mm)
> {
> seqcount_init(&mm->mm_lock_seq);
> @@ -86,11 +87,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
> do_raw_write_seqcount_end(&mm->mm_lock_seq);
> }
>
> -#else
> +static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
> +{
> + /*
> + * Since mmap_lock is a sleeping lock, and waiting for it to become
> + * unlocked is more or less equivalent with taking it ourselves, don't
> + * bother with the speculative path if mmap_lock is already write-locked
> + * and take the slow path, which takes the lock.
> + */
> + return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
> +}
> +
> +static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> +{
> + return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
> +}
> +
> +#else /* CONFIG_PER_VMA_LOCK */
> +
> static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
> -#endif
> +
> +static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
> +{
> + return false;
> +}
> +
> +static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> +{
> + return true;
> +}
> +
> +#endif /* CONFIG_PER_VMA_LOCK */
>
> static inline void mmap_init_lock(struct mm_struct *mm)
> {
> --
> 2.47.0.338.g60cca15819-goog
>
>
