From nobody Fri Sep 18 13:34:36 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 4hmYWc6KQvz6sT37 for ; Fri, 18 Sep 2026 13:34:36 +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 client-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "mxrelay.nyi.freebsd.org", Issuer "YR2" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id 4hmYWc48c3z4ZpV for ; Fri, 18 Sep 2026 13:34:36 +0000 (UTC) (envelope-from git@FreeBSD.org) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1789738476; 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=hWb9g4OUAg48M94+0At7bxY8e4EkDFptWXMRS2Yq7Eg=; b=LQtimjwf3T6lzkEXigB8Q+jJloBQ1PURF2s2P7KUof7j3hnRtD5nCZRhwyfEDQ31X9gbLM mv67vfTeZhIvI+lCXTvkEOor9SzQNJ5hOEnsFJuaWPj3bhxRXYBOJJPTN5Ql8KdH2z7omg I02CMIsdspPfj+TIY5XAm6b+vfoEztnsACeokcC//pGxuLrVAy/+sEd1VKeG2X76M9s7N1 A9xEs/UW9PMtzUPwhkSYIkNr/KUr4Rg4InD0V809/MX7SaB7FZB0BXJCRdsSC/LZI8IJ6T uu9BsjPPMbJuuxpUOczgw2vXFGVU1Pi8UymNY3Tek+GsRw5y8B4ohuFmPV3UnQ== ARC-Seal: i=1; a=rsa-sha256; d=freebsd.org; s=dkim; cv=none; t=1789738476; b=EJzXW9qnP+DhwMzyEomFDx3q8rId/18fp87Wk8Fh7Ou41lyAVKZ4h1aFRNyOIQHxKlQ8k5 17PHhhI3S5wjrhvW6161RKORwWrzwRdPWvNE76Z5m7aGuTtABq0y36seXvRNNwpvD8tTfr 67h7gkLGLshhOEbDPQs1R5DMQZdh0hZIAq8cjHPsAqo5dfKEHCpXbmiycmiEFBvLNBXDsr eA7I2lruUaf5MdY0Jg/Z+hPZaoOxxs3JGT68tVGrE5q6ysX+52NPY/o5MV0iSxML68X8EY ris9CYia37T/M0CvbP8mHDSmOZJ4sB3ucWYQSKGjSSE3P9uvtP13m3ogxhPr8g== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1789738476; 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=hWb9g4OUAg48M94+0At7bxY8e4EkDFptWXMRS2Yq7Eg=; b=Wrr5mPo6Ui4dJ8YRDPr2/PXohJY88UZY/ESfuuqnqWUz9KNsHuld92stk8ecQ/OeOpmCRb XyJukQLL9lz9BtQWfhDjzZqmJyKKQ5Q7Wu7juQhuHRCjpAnu0wKbqP/g3YeMMBEctGniPA f+NP7sSltvF/HG2M7sFwPL6UeRsWimD5Ab8t1KVuaG/at1c5OQ5JDOeQv+JnDrIPtco6Bb q7pdrQFR7nmzjK0zCkXD1svTSei5tHwIGQuS6ubwlWHR2Z6mx1UCtwM78Ew79vXY9MtZJX +Txvzz+asA8G43jNjScWgFY4LaSRzL05aSxzKyAcHkm0Gnu4lEvmfEZlNd8AYQ== 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 4hmYWc3D9kzLFt for ; Fri, 18 Sep 2026 13:34:36 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from git (uid 1279) (envelope-from git@FreeBSD.org) id 3ec61 by gitrepo.freebsd.org (DragonFly Mail Agent v0.13+ on gitrepo.freebsd.org); Fri, 18 Sep 2026 13:34:36 +0000 To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org From: Piotr Kubaj Subject: git: 72ed4d012ea9 - stable/15 - umtx: do not sleep on an unowned mutex after a spurious CAS failure List-Id: Commits to the stable branches of the FreeBSD src repository List-Archive: https://lists.freebsd.org/archives/dev-commits-src-branches List-Help: List-Post: List-Subscribe: List-Unsubscribe: X-BeenThere: dev-commits-src-branches@freebsd.org Sender: owner-dev-commits-src-branches@FreeBSD.org List-Id: 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: pkubaj X-Git-Repository: src X-Git-Refname: refs/heads/stable/15 X-Git-Reftype: branch X-Git-Commit: 72ed4d012ea9a0ad66848f813360b0bcf20c213e Auto-Submitted: auto-generated Date: Fri, 18 Sep 2026 13:34:36 +0000 Message-Id: <6aad3dec.3ec61.467631@gitrepo.freebsd.org> The branch stable/15 has been updated by pkubaj: URL: https://cgit.FreeBSD.org/src/commit/?id=72ed4d012ea9a0ad66848f813360b0bcf20c213e commit 72ed4d012ea9a0ad66848f813360b0bcf20c213e Author: Piotr Kubaj AuthorDate: 2026-09-03 14:08:45 +0000 Commit: Piotr Kubaj CommitDate: 2026-09-18 13:34:25 +0000 umtx: do not sleep on an unowned mutex after a spurious CAS failure On ll/sc architectures casueword32() may report a spurious store-conditional failure (reservation lost to an interrupt, preemption, or another CPU touching the same reservation granule), and this is indistinguishable from a genuine comparison mismatch: both return 1. That is intentional since D20772 and documented in casueword(9) ("The store can fail on load-linked/store-conditional architectures."), so callers must cope. do_lock_normal() does not fully cope. When the initial UMUTEX_UNOWNED -> id acquire CAS fails spuriously, the observed owner is still UMUTEX_UNOWNED, so neither the UMUTEX_CONTESTED branch nor the real-owner case applies, and execution falls through past the "rv == 1 but not contested, likely store failure" comment into the sleep path. There, the contested-bit CAS (expecting the observed owner, i.e. UMUTEX_UNOWNED) succeeds because the mutex really is unowned, stamping m_owner = UMUTEX_CONTESTED with no owner tid, and the thread sleeps on "umtxn" forever: nobody owns the mutex, so no unlock and no wakeup ever arrive. In _UMUTEX_TRY mode the same situation returns a false EBUSY for a free mutex. Treat an observed owner of UMUTEX_UNOWNED like UMUTEX_CONTESTED: try to acquire the mutex, setting the contested bit, instead of falling through to the sleep path. rv == 1 with the observed value equal to the expected value can only mean a spurious store failure, so the mutex is free. If the acquire CAS fails again, the outer loop restarts and re-evaluates ownership. The contested bit set with no waiters present only costs the matching unlock one trip through the kernel. This was hit in practice on powerpc64le (POWER9): the Swift runtime's Synchronization.Mutex issues _umtx_op(UMTX_OP_MUTEX_LOCK) directly with no userspace fast path, so an uncontended lock of an unowned mutex runs the kernel CAS exactly where a spurious failure deadlocks (single-threaded process parked on "umtxn" with m_owner == 0x80000000, observed as Foundation.Process.run() hanging). libthr mostly masks the bug because pthread_mutex_lock() enters the kernel only when there is a real owner that will eventually issue a wakeup. The mechanism was confirmed with an experimental powerpc kernel that instead retried the ll/sc sequence inside casueword32()/casueword(); that also eliminated the hang, but is not proposed here since the single-attempt semantics of casueword(9) are intentional. Reviewed by: kib MFC after: 2 weeks Differential Revision: https://reviews.freebsd.org/D59338 (cherry picked from commit c5d9485e7d7faeba1019301ed83a781dac2d0cb0) --- sys/kern/kern_umtx.c | 20 ++++++++++++-------- 1 file changed, 12 insertions(+), 8 deletions(-) diff --git a/sys/kern/kern_umtx.c b/sys/kern/kern_umtx.c index ab6e578ab380..77ce0f97bb4c 100644 --- a/sys/kern/kern_umtx.c +++ b/sys/kern/kern_umtx.c @@ -1423,19 +1423,23 @@ do_lock_normal(struct thread *td, struct umutex *m, uint32_t flags, } /* - * If no one owns it but it is contested try - * to acquire it. + * If no one owns it, but it is contested or + * the CAS above failed spuriously (possible + * on ll/sc architectures), try to acquire it. + * Sleeping would be forever in the spurious + * case: no owner exists to wake us. */ MPASS(rv == 1); - if (owner == UMUTEX_CONTESTED) { - rv = casueword32(&m->m_owner, - UMUTEX_CONTESTED, &owner, - id | UMUTEX_CONTESTED); + if (owner == UMUTEX_CONTESTED || + owner == UMUTEX_UNOWNED) { + rv = casueword32(&m->m_owner, owner, + &owner, id | UMUTEX_CONTESTED); /* The address was invalid. */ if (rv == -1) return (EFAULT); if (rv == 0) { - MPASS(owner == UMUTEX_CONTESTED); + MPASS(owner == UMUTEX_CONTESTED || + owner == UMUTEX_UNOWNED); return (0); } if (rv == 1) { @@ -1451,7 +1455,7 @@ do_lock_normal(struct thread *td, struct umutex *m, uint32_t flags, continue; } - /* rv == 1 but not contested, likely store failure */ + /* rv == 1 with a real owner, fall through to sleep. */ rv = thread_check_susp(td, false); if (rv != 0) return (rv);