Uprobe srcu fast#12709
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Add a guard(srcu_fast_updown) definition for scoped SRCU-fast-updown read-side critical sections, following the existing pattern of guard(srcu) and guard(srcu_fast). Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
uretprobes_srcu currently uses normal SRCU, which issues two smp_mb() per read lock/unlock pair. This overhead is paid on every uretprobe hit in prepare_uretprobe(). Switch to SRCU-fast-updown, which eliminates the per-reader memory barriers by moving the ordering cost to the grace-period side (synchronize_rcu() instead of smp_mb()). This is acceptable because grace periods (uprobe unregistration) are infrequent compared to reader-side uretprobe hits. The updown flavor is required because the SRCU read lock is acquired in prepare_uretprobe() but can be released from a different context: ri_timer() (timer callback) or dup_utask() (fork path). The srcu_down_read_fast() / srcu_up_read_fast() API is designed for this cross-context pattern and does not carry lockdep tracking that would warn on cross-task usage. For the same-context short-lived SRCU sections in ri_timer() and dup_utask(), guard(srcu_fast_updown) provides proper lockdep coverage. Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
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