From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 80C37396EC79; Mon, 8 Feb 2021 12:12:34 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 80C37396EC79 From: "cvs-commit at gcc dot gnu.org" To: gdb-prs@sourceware.org Subject: [Bug tdep/27369] ARC: Stepping over atomic instruction sequences loops infinitely Date: Mon, 08 Feb 2021 12:12:34 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gdb X-Bugzilla-Component: tdep X-Bugzilla-Version: HEAD X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: cvs-commit at gcc dot gnu.org X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Resolution: X-Bugzilla-Priority: P2 X-Bugzilla-Assigned-To: unassigned at sourceware dot org X-Bugzilla-Target-Milestone: 10.2 X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://sourceware.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gdb-prs@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-prs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 08 Feb 2021 12:12:34 -0000 https://sourceware.org/bugzilla/show_bug.cgi?id=3D27369 --- Comment #2 from cvs-commit at gcc dot gnu.org --- The gdb-10-branch branch has been updated by Shahab Vahedi : https://sourceware.org/git/gitweb.cgi?p=3Dbinutils-gdb.git;h=3Dd8979e81fdae= 49a0a3d5ba484ef870f54e2f750d commit d8979e81fdae49a0a3d5ba484ef870f54e2f750d Author: Shahab Vahedi Date: Thu Oct 31 17:33:08 2019 +0100 gdb: Do not interrupt atomic sequences for ARC When stepping over thread-lock related codes (in uClibc), the inferior process gets stuck and never manages to enter the critical section: ------8<------- 1 size_t fwrite(const void * __restrict ptr, size_t size, 2 size_t nmemb, register FILE * __restrict stream) 3 { 4 size_t retval; 5 __STDIO_AUTO_THREADLOCK_VAR; 6 7 > __STDIO_AUTO_THREADLOCK(stream); 8 9 retval =3D fwrite_unlocked(ptr, size, nmemb, stream); 10 11 __STDIO_AUTO_THREADUNLOCK(stream); 12 13 return retval; 14 } ------>8------- Here, we are at line 7. Using the "next" command leads no where. However, setting a breakpoint on line 9 and issuing "continue" works. Looking at the assembly instructions reveals that we're dealing with the critical section entry code [1] that should never be interrupted, in th= is case by the debugger's implicit breakpoints: ------8<------- ... 1 add_s r0,r13,0x38 2 mov_s r3,1 3 llock r2,[r0] <-. 4 brne.nt r2,0,14 --. | 5 scond r3,[r0] | | 6 bne -10 --|--' 7 brne_s r2,0,84 <-' ... ------>8------- Lines 3 until 5 (inclusive) are supposed to be executed atomically. Therefore, GDB should never (implicitly) insert a breakpoint on lines 4 and 5, else the program will try to acquire the lock again by jumping back to line 3 and gets stuck in an infinite loop. The solution is to make GDB aware of these patterns so it inserts breakpoints after the sequence -- line 6 in this example. [1] =20=20=20 https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/tree/libc/sysdeps/linux/a= rc/bits/atomic.h#n46 ------8<------- ({ = \ __typeof(oldval) prev; = \ = \ __asm__ __volatile__( = \ "1: llock %0, [%1] \n" = \ " brne %0, %2, 2f \n" = \ " scond %3, [%1] \n" = \ " bnz 1b \n" = \ "2: \n" = \ : "=3D&r"(prev) = \ : "r"(mem), "ir"(oldval), = \ "r"(newval) /* can't be "ir". scond can't take limm for "b" *= /\ : "cc", "memory"); = \ = \ prev; = \ }) ------>8------- "llock" (Load Locked) loads the 32-bit word pointed by the source operand. If the load is completed without any interruption or exception, the physical address is remembered, in Lock Physical Address (LPA), and the Lock Flag (LF) is set to 1. LF is a non-architecturally visible flag and is cleared whenever an interrupt or exception takes place. LF is also cleared (atomically) whenever another process writes to the LPA. "scond" (Store Conditional) will write to the destination address if and only if the LF is set to 1. When finished, with or without a write, it atomically copies the LF value to ZF (Zero Flag). These two instructions together provide the mechanism for entering a critical section. The code snippet above comes from uClibc: ----------------------- v3 (after Tom's remarks[2]): handle_atomic_sequence() - no need to initialize the std::vector with "{}" - fix typo in comments: "conditial" -> "conditional" - add braces to the body of "if" condition because of the comment line arc_linux_software_single_step() - make the performance slightly more efficient by moving a few variables after the likely "return" point. v2 (after Simon's remarks[3]): - handle_atomic_sequence() gets a copy of an instruction instead of a reference. - handle_atomic_sequence() asserts if the given instruction is an llock. [2] https://sourceware.org/pipermail/gdb-patches/2021-February/175805.html [3] https://sourceware.org/pipermail/gdb-patches/2021-January/175487.html gdb/ChangeLog: PR tdep/27369 * arc-linux-tdep.c (handle_atomic_sequence): New. (arc_linux_software_single_step): Call handle_atomic_sequence(). --=20 You are receiving this mail because: You are on the CC list for the bug.=