From nobody Tue Oct 27 01:18:35 2026 X-Original-To: dev-commits-src-branches@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 4jDCLM4zxcz6wFvH for ; Tue, 27 Oct 2026 01:18:35 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from mxrelay.nyi.freebsd.org (mxrelay.nyi.freebsd.org [IPv6:2610:1c1:1:606c::19:3]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange x25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mx1.freebsd.org (Postfix) with ESMTPS id 4jDCLM4DsRz3Cm5 for ; Tue, 27 Oct 2026 01:18:35 +0000 (UTC) (envelope-from git@FreeBSD.org) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1793063915; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=tIgedSS+zwVZ5CJryHCO0CEfJOzKbX9sFVq652YciR4=; b=IZndda9J9I3GYZ2vt+ftsLkyWhSugQiEw4iQc3hPDgda6OufdsqeUOt5CUSLXs4FxRttB6 BhtPQWphDkk1K0K7UGg9YQj8qed93b9yDznLNh5gxvufXyAiJI7mjgkecx7GLDrtmSzUK8 YnXbTCnLKwlATHNAUoI0qnkahXddJt5u9ASgb6tAHRMYbXGQfLx1Bfm9JQIJwnB2nNMUPW Y/++U64NM+RNhetCYy6NIFpX6B4ULRiLWPMccWW7lmURnhOS7Xf7WwjpADSWvitFjElnfC d8uaO7SXgBAn5txUTYt06GCQAkZLRkFWByBJr8ZA0cFvEQV+U2KuWd8nhqCnGQ== ARC-Seal: i=1; a=rsa-sha256; d=freebsd.org; s=dkim; cv=none; t=1793063915; b=T//S+Vs9+gYe24RH+mkKAJuCqW2QX532KaRzny4j2zb8vobJU21b94vEwYO0UWzx5QPY10 hpT4tYNIvh2JSaxdyLKDsUtyBcBxkyuDnW/MiddO2IfOG6q+WeN9jw7HzgCxD0R5KUWIXg Wneq6HL5pm+is0+Ds3P4h31ao8AUrpVCFw6N/uOn/O4tFeD/DVLfO5alu/sl9PbhNlgATN JePTcetw593JkYZSTWTYBPwFp4aqfHL74/1KVOmAgPaYpUzobEuflEmynfoRzxjzV0k3yg rF/Te8sy+4ghNKOL81+FAD8DUaASQ+0yM0c7LUp1NzvKVWKJeNw+AUb2perJJQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1793063915; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=tIgedSS+zwVZ5CJryHCO0CEfJOzKbX9sFVq652YciR4=; b=tqjRgGMfcs7xXhujRfF+HLizWfQEc+c/G6CFZ0i3KPpCz970gXBvWDU125RwyxfKJf+d32 9VjsYHPbCIHpDtUjGK6GkBuIEXVCWXD1d5ztB3CNQjtkyew//w6cItyUxsrT4608V9eeO1 3/uGEvryDKvo4SO+51xmpGSStpoqXn+FHpJOXUFpUhOopFgsqTkIn0Y088iDgnILtllSWB NBzhnDcJYU5HKA9857GIEKftq1tWwh0kMBOVZ+QuQ5jDvrUAs+1rZKvj32tOR+2Gr88Oi1 K0z2n9XTrihyMdgEusPvSVtEk/KyFnQhSxrG8I0JiwnMR+/Lvz+tGvwfheBy3g== ARC-Authentication-Results: i=1; mx1.freebsd.org; none Received: from gitrepo.freebsd.org (gitrepo.freebsd.org [IPv6:2610:1c1:1:6068::e6a:5]) by mxrelay.nyi.freebsd.org (Postfix) with ESMTP id 4jDCLM3vrDzyJS for ; Tue, 27 Oct 2026 01:18:35 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from git (uid 1279) (envelope-from git@FreeBSD.org) id 3412a by gitrepo.freebsd.org (DragonFly Mail Agent v0.13+ on gitrepo.freebsd.org); Tue, 27 Oct 2026 01:18:35 +0000 To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org From: Minsoo Choo Subject: git: a2b34aaa8d80 - stable/14 - powerpc: Avoid deadlock when CPUs exchange threads List-Archive: https://lists.freebsd.org/archives/dev-commits-src-branches X-BeenThere: dev-commits-src-branches@freebsd.org Sender: owner-dev-commits-src-branches@FreeBSD.org List-Id: Commits to the stable branches of the FreeBSD src repository List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Git-Committer: mchoo X-Git-Repository: src X-Git-Refname: refs/heads/stable/14 X-Git-Reftype: branch X-Git-Commit: a2b34aaa8d805782b8786e2deb90527bb49d99b0 Auto-Submitted: auto-generated Date: Tue, 27 Oct 2026 01:18:35 +0000 Message-Id: <6adffbeb.3412a.1d0a8c6a@gitrepo.freebsd.org> The branch stable/14 has been updated by mchoo: URL: https://cgit.FreeBSD.org/src/commit/?id=a2b34aaa8d805782b8786e2deb90527bb49d99b0 commit a2b34aaa8d805782b8786e2deb90527bb49d99b0 Author: Minsoo Choo AuthorDate: 2026-09-16 19:37:01 +0000 Commit: Minsoo Choo CommitDate: 2026-10-27 01:18:14 +0000 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)