Re: [PATCH] uprobes: avoid postponing return-probe SRCU lease expiration

[sashiko-bot@kernel.org]

> prepare_uretprobe() reprograms the per-task return-instance timer on every
> successfully installed return probe. If a traced outer function remains
> active while nested traced calls arrive more frequently than the timeout,
> the shared timer keeps moving forward and the outer instance can hold its
> SRCU read-side lease indefinitely. This delays unrelated synchronous uprobe
> unregister operations that must wait for the same SRCU grace period.
> 
> Use timer_reduce() to preserve the earliest pending expiry. Pair a full
> memory barrier after publishing a return instance with one before the timer
> callback traverses the list. A publisher that observes the earlier timer
> as pending must then be visible to that callback - otherwise the producer
> rearms the expired timer.
> 
> In an x86-64 QEMU reproducer with nested traced calls every 10 ms, an
> unrelated tracefs event disable took 7,740 ms on the baseline and 70 ms
> with the change. A corrected monotonic-clock regression run measured
> 2,210 ms versus 79 ms, with 15 additional long-lived timer-boundary
> cases and process exit/exec checks passing. Narrow LKMM store-buffering
> litmus tests observed the lost-rearm outcome without both barriers and
> forbade it when both were present.
> 
> Fixes: dd1a7567784e ("uprobes: SRCU-protect uretprobe lifetime (with timeout)")
> Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261115155921.260878-1-thnkslprpt@gmail.com?part=1