From nobody Fri Sep 04 09:27:57 2026 X-Original-To: dev-commits-src-all@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 4hbrjZ6tWQz6qNH8 for ; Fri, 04 Sep 2026 09:28:02 +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 4hbrjZ4n8Sz40dv for ; Fri, 04 Sep 2026 09:28:02 +0000 (UTC) (envelope-from git@FreeBSD.org) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1788514082; 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=FrOGnTUIiAn8i/ETUwPzUPNkZTDdw8u1nVus6XoaNF0=; b=VR646SBATrg6ppLjCHcGcjleH/t/4f8OYnpMAPHCMMABPtk76afv166zoir3R9e+XXeZGE G+/yHf0QyYRFKczJvxHoTWup2loEjaIWQ+93QQDUQ9dBNI37fBTFFcjy25n/AHGHIuvoJS P+gZeMZtLhhntYgWd53FulW+nh+jx4BcGBHFIHC7s4tqqWCmNfY02eJg79Kh2L+17l1Kvy 6k1zC2Fh/NZZZZ9m0sTsbjcguj4690CN14dmla65K0iIvpDpzSs0NHQZPDWxv/3ES9UVfv C+u7QTVAL/8iS/pnJUyxuTWe12y0czMBHeUbyP0URktiouaFe4vrmMSCwEjIgw== ARC-Seal: i=1; s=dkim; d=freebsd.org; t=1788514082; a=rsa-sha256; cv=none; b=W9HjOEhfRo+G4RPXssjkL0+5AaSW7VsoYh8Kdw23FwapnxpahWQZdcn2fNl5XdoE93vi2b 6t84wCXXmR3ryA0sJYVIUHgfyn2oU9G1ENcLVOqU2ljBjD87uzJ2x+olB+jRCNx/Ep2LUb UevZVT16chRl6jnv9hRFiQbChU79wIPx6aSLs51J2MSmOQkpRDXSnkcDrEVLArnoJOT1EM OPDKszh3rcOhoBIidjQoGZaxwCXqNF9NYjKbj3jjyWNtQJj6IZy499+gtIzbhc99SJqvu6 q5GSB3vw2h3kf08Ltz1pcKW06VmuXbttToxVWjIb+/gNmCbd7BDAwxnsXwjpbg== ARC-Authentication-Results: i=1; mx1.freebsd.org; none ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1788514082; 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=FrOGnTUIiAn8i/ETUwPzUPNkZTDdw8u1nVus6XoaNF0=; b=v0PdKqsEsRQBTZm24dfydVVty/cDhdl9c8yYsUXNBZmIxvdvV/+eu+HynwtQoGWTCehWmR X1YTNA/4jAGWjWH6k2VNnaBdMj2hA037um3HSh1JDPL2bLi6BjfUW2hjt7WRLWXT5LuCgW PnKLHFaqDrRDTXcEfv0igSpxQmY9bhZusKYEGv7qlQXOWQbAMZtEvuIBXiz5WNsSPv4uba 3g0iN3Rj3jUWediHPiLwia91a/X54bOr8S/yLhaGZFBIAfR0xhGHXEw/uxuEJqhGdTeec2 yTq9zNft8Xb3XWrVDY8gO2IgK/7XGXCehJqHJiBv8pyASPi/MtvdvT1F3zciXQ== Received: from gitrepo.freebsd.org (gitrepo.freebsd.org [IPv6:2610:1c1:1:6068::e6a:5]) by mxrelay.nyi.freebsd.org (Postfix) with ESMTP id 4hbrjZ2Z5ZztbF for ; Fri, 04 Sep 2026 09:28:02 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from git (uid 1279) (envelope-from git@FreeBSD.org) id 3e11f by gitrepo.freebsd.org (DragonFly Mail Agent v0.13+ on gitrepo.freebsd.org); Fri, 04 Sep 2026 09:27:57 +0000 To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org From: Piotr Kubaj Subject: git: c5d9485e7d7f - main - umtx: do not sleep on an unowned mutex after a spurious CAS failure List-Id: Commit messages for all branches of the src repository List-Archive: https://lists.freebsd.org/archives/dev-commits-src-all List-Help: List-Post: List-Subscribe: List-Unsubscribe: X-BeenThere: dev-commits-src-all@freebsd.org Sender: owner-dev-commits-src-all@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/main X-Git-Reftype: branch X-Git-Commit: c5d9485e7d7faeba1019301ed83a781dac2d0cb0 Auto-Submitted: auto-generated Date: Fri, 04 Sep 2026 09:27:57 +0000 Message-Id: <6a9a8f1d.3e11f.1fc3abec@gitrepo.freebsd.org> The branch main has been updated by pkubaj: URL: https://cgit.FreeBSD.org/src/commit/?id=c5d9485e7d7faeba1019301ed83a781dac2d0cb0 commit c5d9485e7d7faeba1019301ed83a781dac2d0cb0 Author: Piotr Kubaj AuthorDate: 2026-09-03 14:08:45 +0000 Commit: Piotr Kubaj CommitDate: 2026-09-04 09:27:49 +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 --- 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 ac2469379690..55ef536d5b98 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);