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definitions=main-2609240023 On 9/24/26 11:10, sashiko-bot@kernel.org wrote: > Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: > - [Medium] ftrace: time_max TOCTOU data race clobbers maximum latency spikes > -- > > commit 9fb82451733a5ec95aac95499945b5c01e39d82b > Author: Yun Zhou > > ftrace: add max field to function profiler stats > > This commit adds a max field to the function profiler statistics to record the > maximum single-call duration. This allows users to better investigate > occasional latency spikes when reviewing function execution times. > >> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c >> index 53d5db60bfa56..dfd25e3c87f71 100644 >> --- a/kernel/trace/ftrace.c >> +++ b/kernel/trace/ftrace.c > [ ... ] >> @@ -920,6 +923,8 @@ static void profile_graph_return(struct ftrace_graph_ret *trace, >> if (rec) { >> rec->time += calltime; >> rec->time_squared += calltime * calltime; >> + if (calltime > rec->time_max) >> + rec->time_max = calltime; > > [Severity: Medium] > Could this non-atomic check and update open a time-of-check to time-of-use > data race against concurrent nested interrupt context tracing on the same CPU? > > Since profile_graph_return() is executed with local interrupts enabled, an > interrupt or NMI could fire exactly between the condition check and the > assignment. If the interrupt handler traces the identical function and > records a massive latency spike, it would update rec->time_max. When the > interrupt returns, the normal context resumes and executes the assignment, > overwriting the true maximum latency spike with its smaller value. > > Because ftrace explicitly permits tracing recursion from different contexts > like hardirq or NMI, does this require an atomic compare-and-exchange loop > or similar protection to avoid dropping the genuine latency spikes this > patch intends to capture? > I think this is intentional and consistent with the profiler's design. profile_graph_return() runs under preempt_notrace() and the existing time, time_squared and counter updates are all likewise non-atomic best- effort accounting; the recursion guard only protects record allocation, not the stat updates. This is a hot path - every traced function return passes through it, and keeping the observation overhead minimal is more important here than never dropping a single sample.