On 11/23/21 1:53 PM, Darrick J. Wong wrote:
From: Darrick J. Wong <djwong@xxxxxxxxxx>
In commit de555f66, we converted the atomic64_t implementation to use
the liburcu uatomic_* functions. Regrettably, nobody tried to build
xfsprogs on a 32-bit architecture (hint: maintainers don't scale well
anymore) so nobody noticed that the build fails due to the unknown
symbol _uatomic_link_error. This is what happens when liburcu doesn't
know how to perform atomic updates to a variable of a certain size, due
to some horrid macro magic in urcu.h.
Rather than a strict revert to non-atomic updates for these platforms or
(which would introduce a landmine) or roll everything back for the sake
of older platforms, I went with providing a custom atomic64_t
implementation that uses a single pthread mutex. This enables us to
work around the fact that the kernel atomic64_t API doesn't require a
special initializer function, and is probably good enough since there
are only a handful of atomic64_t counters in the kernel.
Clean up the type declarations of a couple of variables in libxlog to
match the kernel usage, though that's probably overkill.
Eventually we'll want to decide if we're deprecating 32-bit, but this
fixes them in the mean time.
Fixes: de555f66 ("atomic: convert to uatomic")
Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
After a brief sidebar w/ Darrick I think we should change atomic64_add to take
an int64_t, I can fix that on commit. It shouldn't matter for our current usage.
Reviewed-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---
configure.ac | 1 +
include/atomic.h | 46 ++++++++++++++++++++++++++++++++++++++++++++++
include/builddefs.in | 4 ++++
include/libxlog.h | 4 ++--
libxfs/init.c | 4 ++++
m4/package_urcu.m4 | 19 +++++++++++++++++++
repair/phase2.c | 2 +-
7 files changed, 77 insertions(+), 3 deletions(-)
diff --git a/configure.ac b/configure.ac
index 89a53170..6adbee8c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -238,6 +238,7 @@ AC_CHECK_SIZEOF([long])
AC_CHECK_SIZEOF([char *])
AC_TYPE_UMODE_T
AC_MANUAL_FORMAT
+AC_HAVE_LIBURCU_ATOMIC64
AC_CONFIG_FILES([include/builddefs])
AC_OUTPUT
diff --git a/include/atomic.h b/include/atomic.h
index 2804815e..79e58dfe 100644
--- a/include/atomic.h
+++ b/include/atomic.h
@@ -60,11 +60,57 @@ static inline bool atomic_dec_and_lock(atomic_t *a, spinlock_t *lock)
return 0;
}
+#ifdef HAVE_LIBURCU_ATOMIC64
+/*
+ * On most (64-bit) platforms, liburcu can handle 64-bit atomic counter
+ * updates, so we preferentially use that.
+ */
#define atomic64_read(a) uatomic_read(a)
#define atomic64_set(a, v) uatomic_set(a, v)
#define atomic64_add(v, a) uatomic_add(a, v)
#define atomic64_sub(v, a) uatomic_sub(a, v)
#define atomic64_inc(a) uatomic_inc(a)
#define atomic64_dec(a) uatomic_dec(a)
+#else
+/*
+ * If we don't detect support for that, emulate it with a lock. Currently
+ * there are only three atomic64_t counters in userspace and none of them are
+ * performance critical, so we serialize them all with a single mutex since
+ * the kernel atomic64_t API doesn't have an _init call.
+ */
+extern pthread_mutex_t atomic64_lock;
+
+static inline int64_t
+atomic64_read(atomic64_t *a)
+{
+ int64_t ret;
+
+ pthread_mutex_lock(&atomic64_lock);
+ ret = *a;
+ pthread_mutex_unlock(&atomic64_lock);
+ return ret;
+}
+
+static inline void
+atomic64_add(int v, atomic64_t *a)
+{
+ pthread_mutex_lock(&atomic64_lock);
+ (*a) += v;
+ pthread_mutex_unlock(&atomic64_lock);
+}
+
+static inline void
+atomic64_set(atomic64_t *a, int64_t v)
+{
+ pthread_mutex_lock(&atomic64_lock);
+ (*a) = v;
+ pthread_mutex_unlock(&atomic64_lock);
+}
+
+#define atomic64_inc(a) atomic64_add(1, (a))
+#define atomic64_dec(a) atomic64_add(-1, (a))
+#define atomic64_sub(v, a) atomic64_add(-(v), (a))
+
+#endif /* HAVE_URCU_ATOMIC64 */
#endif /* __ATOMIC_H__ */
diff --git a/include/builddefs.in b/include/builddefs.in
index 78eddf4a..9d0b0800 100644
--- a/include/builddefs.in
+++ b/include/builddefs.in
@@ -122,6 +122,7 @@ HAVE_SYSTEMD = @have_systemd@
SYSTEMD_SYSTEM_UNIT_DIR = @systemd_system_unit_dir@
HAVE_CROND = @have_crond@
CROND_DIR = @crond_dir@
+HAVE_LIBURCU_ATOMIC64 = @have_liburcu_atomic64@
GCCFLAGS = -funsigned-char -fno-strict-aliasing -Wall
# -Wbitwise -Wno-transparent-union -Wno-old-initializer -Wno-decl
@@ -159,6 +160,9 @@ endif
LIBICU_LIBS = @libicu_LIBS@
LIBICU_CFLAGS = @libicu_CFLAGS@
+ifeq ($(HAVE_LIBURCU_ATOMIC64),yes)
+PCFLAGS += -DHAVE_LIBURCU_ATOMIC64
+endif
SANITIZER_CFLAGS += @addrsan_cflags@ @threadsan_cflags@ @ubsan_cflags@
SANITIZER_LDFLAGS += @addrsan_ldflags@ @threadsan_ldflags@ @ubsan_ldflags@
diff --git a/include/libxlog.h b/include/libxlog.h
index adaa9963..3ade7ffa 100644
--- a/include/libxlog.h
+++ b/include/libxlog.h
@@ -11,8 +11,8 @@
* the need to define any exotic kernel types in userland.
*/
struct xlog {
- xfs_lsn_t l_tail_lsn; /* lsn of 1st LR w/ unflush buffers */
- xfs_lsn_t l_last_sync_lsn;/* lsn of last LR on disk */
+ atomic64_t l_tail_lsn; /* lsn of 1st LR w/ unflush buffers */
+ atomic64_t l_last_sync_lsn;/* lsn of last LR on disk */
xfs_mount_t *l_mp; /* mount point */
struct xfs_buftarg *l_dev; /* dev_t of log */
xfs_daddr_t l_logBBstart; /* start block of log */
diff --git a/libxfs/init.c b/libxfs/init.c
index 0f7e8950..75ff4d49 100644
--- a/libxfs/init.c
+++ b/libxfs/init.c
@@ -25,6 +25,10 @@
#include "libxfs.h" /* for now */
+#ifndef HAVE_LIBURCU_ATOMIC64
+pthread_mutex_t atomic64_lock = PTHREAD_MUTEX_INITIALIZER;
+#endif
+
char *progname = "libxfs"; /* default, changed by each tool */
struct cache *libxfs_bcache; /* global buffer cache */
diff --git a/m4/package_urcu.m4 b/m4/package_urcu.m4
index f0337f34..f8e798b6 100644
--- a/m4/package_urcu.m4
+++ b/m4/package_urcu.m4
@@ -20,3 +20,22 @@ AC_DEFUN([AC_PACKAGE_NEED_RCU_INIT],
AC_MSG_RESULT(no))
AC_SUBST(liburcu)
])
+
+#
+# Make sure that calling uatomic_inc on a 64-bit integer doesn't cause a link
+# error on _uatomic_link_error, which is how liburcu signals that it doesn't
+# support atomic operations on 64-bit data types.
+#
+AC_DEFUN([AC_HAVE_LIBURCU_ATOMIC64],
+ [ AC_MSG_CHECKING([for atomic64_t support in liburcu])
+ AC_TRY_LINK([
+#define _GNU_SOURCE
+#include <urcu.h>
+ ], [
+ long long f = 3;
+ uatomic_inc(&f);
+ ], have_liburcu_atomic64=yes
+ AC_MSG_RESULT(yes),
+ AC_MSG_RESULT(no))
+ AC_SUBST(have_liburcu_atomic64)
+ ])
diff --git a/repair/phase2.c b/repair/phase2.c
index cb9adf1d..32ffe18b 100644
--- a/repair/phase2.c
+++ b/repair/phase2.c
@@ -128,7 +128,7 @@ zero_log(
* is a v5 filesystem.
*/
if (xfs_sb_version_hascrc(&mp->m_sb))
- libxfs_max_lsn = log->l_last_sync_lsn;
+ libxfs_max_lsn = atomic64_read(&log->l_last_sync_lsn);
}
static bool
