From a2b34aaa8d805782b8786e2deb90527bb49d99b0 Mon Sep 17 00:00:00 2001 From: Minsoo Choo Date: Wed, 16 Sep 2026 15:37:01 -0400 Subject: [PATCH] powerpc: Avoid deadlock when CPUs exchange threads The PowerPC context switch waited for the incoming thread to leave blocked_lock before releasing the outgoing thread. If two CPUs selected each other's running threads concurrently, both CPUs waited for the other to perform the release and neither could progress. This appeared as intermittent boot hangs under QEMU MTTCG when taskqueue threads were bound to opposite CPUs. 8df2e5421468 ("powerpc: put the isync inside the TD_LOCK()...") attempted to fix the same hang by moving isync into the polling loop. That did not address the circular wait: isync cannot cause either CPU to execute the release store after the loop. It only added context synchronization to failed iterations and changed timing. Move isync back to the successful exit, which preserves acquire ordering for subsequent context loads. Match the ordering used by other architectures: publish the outgoing thread's new lock before waiting for the incoming thread. Apply the correction to both 32-bit and 64-bit switch paths. The 2015 change moved the release after the stack switch because the old implementation continued through pmap_activate() and optional state restoration on the outgoing thread's stack after making that thread runnable. The switch path has since changed: after the release it only polls wired td_lock state using registers, then immediately loads the incoming PCB_SP before updating per-CPU state or making any calls. cpu_switch() also runs with external and decrementer interrupts disabled by spinlock_enter(). Thus this does not restore the post-release call window that the 2015 change corrected. Reviewed by: jhb, jhibbits Approved by: jhb (mentor) Fixes: 53607fe3cc8d ("Fix an extremely subtle concurrency bug...") Fixes: 7a49d964d3f9 ("Merge r278429 from ppc64:") Fixes: 8df2e5421468 ("powerpc: put the isync inside the TD_LOCK()...") MFC after: 2 weeks Sponsored by: FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D59742 (cherry picked from commit 0571d8a0cc606be9a216439b3cfea685b709d6ea) --- sys/powerpc/powerpc/swtch32.S | 18 +++++++++++------- sys/powerpc/powerpc/swtch64.S | 18 +++++++++++------- 2 files changed, 22 insertions(+), 14 deletions(-) diff --git a/sys/powerpc/powerpc/swtch32.S b/sys/powerpc/powerpc/swtch32.S index ef1a397bf74d..fd531de42a71 100644 --- a/sys/powerpc/powerpc/swtch32.S +++ b/sys/powerpc/powerpc/swtch32.S @@ -129,6 +129,15 @@ ENTRY(cpu_switch) sync /* Make sure all of that finished */ cpu_switchin: + /* + * Release the old thread before waiting for the new one. Two CPUs + * may exchange threads, so waiting first would leave each CPU waiting + * for the other to perform this store. + */ + cmpwi %r14,0 + beq- 1f + stw %r16,TD_LOCK(%r14) /* ULE: update old thread's lock */ +1: #if defined(SMP) && defined(SCHED_ULE) /* Wait for the new thread to become unblocked */ bl 1f @@ -141,19 +150,14 @@ cpu_switchin: blocked_loop: lwz %r7,TD_LOCK(%r2) cmpw %r6,%r7 - isync beq- blocked_loop + isync /* Acquire thread state. */ #endif lwz %r17,TD_PCB(%r2) /* Get new current PCB */ lwz %r1,PCB_SP(%r17) /* Load new stack pointer */ - /* Release old thread now that we have a stack pointer set up */ - cmpwi %r14,0 - beq- 1f - stw %r16,TD_LOCK(%r14) /* ULE: update old thread's lock */ - -1: mfsprg %r7,0 /* Get the pcpu pointer */ + mfsprg %r7,0 /* Get the pcpu pointer */ stw %r2,PC_CURTHREAD(%r7) /* Store new current thread */ lwz %r17,TD_PCB(%r2) /* Store new current PCB */ stw %r17,PC_CURPCB(%r7) diff --git a/sys/powerpc/powerpc/swtch64.S b/sys/powerpc/powerpc/swtch64.S index ba37274d32bb..2ea97891190e 100644 --- a/sys/powerpc/powerpc/swtch64.S +++ b/sys/powerpc/powerpc/swtch64.S @@ -187,6 +187,15 @@ save_tar: sync /* Make sure all of that finished */ cpu_switchin: + /* + * Release the old thread before waiting for the new one. Two CPUs + * may exchange threads, so waiting first would leave each CPU waiting + * for the other to perform this store. + */ + cmpdi %r14,0 + beq- 1f + std %r16,TD_LOCK(%r14) /* ULE: update old thread's lock */ +1: #if defined(SMP) && defined(SCHED_ULE) /* Wait for the new thread to become unblocked */ addis %r6,%r2,TOC_REF(blocked_lock)@ha @@ -194,20 +203,15 @@ cpu_switchin: blocked_loop: ld %r7,TD_LOCK(%r13) cmpd %r6,%r7 - isync beq- blocked_loop + isync /* Acquire thread state. */ #endif ld %r17,TD_PCB(%r13) /* Get new PCB */ ld %r1,PCB_SP(%r17) /* Load the stack pointer */ addi %r1,%r1,-48 /* Remember about cpu_switch stack frame */ - /* Release old thread now that we have a stack pointer set up */ - cmpdi %r14,0 - beq- 1f - std %r16,TD_LOCK(%r14) /* ULE: update old thread's lock */ - -1: mfsprg %r7,0 /* Get the pcpu pointer */ + mfsprg %r7,0 /* Get the pcpu pointer */ std %r13,PC_CURTHREAD(%r7) /* Store new current thread */ ld %r17,TD_PCB(%r13) /* Store new current PCB */ std %r17,PC_CURPCB(%r7)