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sched/signal: Run a synchronous signal handler out of the critical section. - #20640
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…ction. nxsig_queue_action() runs the signal handler at once when the signal is for the calling thread. It is called from nxsig_tcbdispatch(), which holds the critical section. In a kernel build this happens at the end of each system call with a pending signal: the user handler then runs in user mode with interrupts disabled, and on SMP its thread holds the global critical section lock until the handler returns. The other CPUs wait for the lock all that time, and the CPU of the handler takes no interrupt. Let nxsig_queue_action() only mark the delivery, and deliver in nxsig_tcbdispatch() after it leaves the critical section. A signal that an interrupt queues meanwhile is delivered in the same loop. Assisted-by: Claude Code:claude-opus-5-5 Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
xiaoxiang781216
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Oct 13, 2026
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acassis
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Oct 15, 2026
This was referenced Oct 15, 2026
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Summary
When a signal with a handler is for the calling thread,
nxsig_queue_action()runs the handler at once. It is called fromnxsig_tcbdispatch(), which holds the critical section. So the handler runs with interrupts disabled, and on SMP its thread holds the global critical section lock until the handler returns.In a protected or kernel build this happens at the end of each system call that leaves a pending signal unmasked: the user handler runs in user mode with interrupts disabled. Its CPU takes no interrupt, and the other CPUs wait for the critical section lock until the handler returns.
This PR lets
nxsig_queue_action()only mark the delivery.nxsig_tcbdispatch()delivers after it leaves the critical section. A signal that an interrupt queues meanwhile is delivered in the same loop.Impact
A handler that runs at once for the calling thread now runs with interrupts enabled and without the critical section. It still runs before
nxsig_tcbdispatch()returns, soraise()andkill()of the calling thread behave as before.Not changed here:
nxsig_unmask_pendingsignal()holdssched_lock()while it dispatches, so such a handler still runs with preemption disabled.Testing
qemu-armv8a:knsh, 1 CPU, withCONFIG_SCHED_IRQMONITOR=y. A test program blocks SIGUSR1, sends it to itself and unblocks it, so the handler runs at the end ofsigprocmask(). The handler spins for about 50 ms and reads/proc/irqsbefore and after (a read returns the interrupts since the last read). Over 100 handlers:ostestexits with status 0 onqemu-armv8a:knshand onqemu-armv8a:nsh(flat build). Build:lm3s6965-ek:qemu-flat, no warnings.