Re: [PATCH 23.11] rcu: fix implicit conversion in bit shift

[Xueming Li <xuemingl@nvidia.com>]

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Hi Andre,

Thanks for your help, patch enqueued to 23.11 LTS patch list.

Regards,
Xueming

________________________________
From: Andre Muezerie <andremue@linux.microsoft.com>
Sent: Monday, December 9, 2024 11:56 PM
To: stable@dpdk.org <stable@dpdk.org>
Cc: Andre Muezerie <andremue@linux.microsoft.com>
Subject: [PATCH 23.11] rcu: fix implicit conversion in bit shift

[ upstream commit ffe827f38e6e0be8a307d7ef9c0e1347874f0af7 ]

../lib/rcu/rte_rcu_qsbr.c(101): warning C4334: '<<': result of 32-bit
 shift implicitly converted to 64 bits (was 64-bit shift intended?)
../lib/rcu/rte_rcu_qsbr.c(107): warning C4334: '<<': result of 32-bit
 shift implicitly converted to 64 bits (was 64-bit shift intended?)
../lib/rcu/rte_rcu_qsbr.c(145): warning C4334: '<<': result of 32-bit
 shift implicitly converted to 64 bits (was 64-bit shift intended?)

These warnings are being issued by the MSVC compiler. Since the result is
being stored in a variable of type uint64_t, it makes sense to shift a
64-bit number instead of shifting a 32-bit number and then having the
compiler to convert the result implicitly to 64 bits.
UINT64_C was used in the fix as it is the portable way to define a 64-bit
constant (ULL suffix is architecture dependent).

From reading the code this is also a bugfix:
(1 << id), where id = thread_id & 0x3f, was wrong when thread_id > 0x1f.

Signed-off-by: Andre Muezerie <andremue@linux.microsoft.com>
---
 lib/rcu/rte_rcu_qsbr.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/lib/rcu/rte_rcu_qsbr.c b/lib/rcu/rte_rcu_qsbr.c
index 41a44be4b9..e46ce7958e 100644
--- a/lib/rcu/rte_rcu_qsbr.c
+++ b/lib/rcu/rte_rcu_qsbr.c
@@ -104,11 +104,11 @@ rte_rcu_qsbr_thread_register(struct rte_rcu_qsbr *v, unsigned int thread_id)
         /* Check if the thread is already registered */
         old_bmap = rte_atomic_load_explicit(__RTE_QSBR_THRID_ARRAY_ELM(v, i),
                                         rte_memory_order_relaxed);
-       if (old_bmap & 1UL << id)
+       if (old_bmap & RTE_BIT64(id))
                 return 0;

         do {
-               new_bmap = old_bmap | (1UL << id);
+               new_bmap = old_bmap | RTE_BIT64(id);
                 success = rte_atomic_compare_exchange_strong_explicit(
                                         __RTE_QSBR_THRID_ARRAY_ELM(v, i),
                                         &old_bmap, new_bmap,
@@ -117,7 +117,7 @@ rte_rcu_qsbr_thread_register(struct rte_rcu_qsbr *v, unsigned int thread_id)
                 if (success)
                         rte_atomic_fetch_add_explicit(&v->num_threads,
                                                 1, rte_memory_order_relaxed);
-               else if (old_bmap & (1UL << id))
+               else if (old_bmap & RTE_BIT64(id))
                         /* Someone else registered this thread.
                          * Counter should not be incremented.
                          */
@@ -156,11 +156,11 @@ rte_rcu_qsbr_thread_unregister(struct rte_rcu_qsbr *v, unsigned int thread_id)
         /* Check if the thread is already unregistered */
         old_bmap = rte_atomic_load_explicit(__RTE_QSBR_THRID_ARRAY_ELM(v, i),
                                         rte_memory_order_relaxed);
-       if (!(old_bmap & (1UL << id)))
+       if (!(old_bmap & RTE_BIT64(id)))
                 return 0;

         do {
-               new_bmap = old_bmap & ~(1UL << id);
+               new_bmap = old_bmap & ~RTE_BIT64(id);
                 /* Make sure any loads of the shared data structure are
                  * completed before removal of the thread from the list of
                  * reporting threads.
@@ -173,7 +173,7 @@ rte_rcu_qsbr_thread_unregister(struct rte_rcu_qsbr *v, unsigned int thread_id)
                 if (success)
                         rte_atomic_fetch_sub_explicit(&v->num_threads,
                                                 1, rte_memory_order_relaxed);
-               else if (!(old_bmap & (1UL << id)))
+               else if (!(old_bmap & RTE_BIT64(id)))
                         /* Someone else unregistered this thread.
                          * Counter should not be incremented.
                          */
@@ -234,7 +234,7 @@ rte_rcu_qsbr_dump(FILE *f, struct rte_rcu_qsbr *v)
                         t = rte_ctz64(bmap);
                         fprintf(f, "%u ", id + t);

-                       bmap &= ~(1UL << t);
+                       bmap &= ~RTE_BIT64(t);
                 }
         }

@@ -261,7 +261,7 @@ rte_rcu_qsbr_dump(FILE *f, struct rte_rcu_qsbr *v)
                                 rte_atomic_load_explicit(
                                         &v->qsbr_cnt[id + t].lock_cnt,
                                         rte_memory_order_relaxed));
-                       bmap &= ~(1UL << t);
+                       bmap &= ~RTE_BIT64(t);
                 }
         }

--
2.47.0.vfs.0.3


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