io-mapping: don't disable preempt on RT in io_mapping_map_atomic_wc().

io_mapping_map_atomic_wc() disables preemption and pagefaults for
historical reasons.  The conversion to io_mapping_map_local_wc(), which
only disables migration, cannot be done wholesale because quite some call
sites need to be updated to accommodate with the changed semantics.

On PREEMPT_RT enabled kernels the io_mapping_map_atomic_wc() semantics are
problematic due to the implicit disabling of preemption which makes it
impossible to acquire 'sleeping' spinlocks within the mapped atomic
sections.

PREEMPT_RT replaces the preempt_disable() with a migrate_disable() for
more than a decade.  It could be argued that this is a justification to do
this unconditionally, but PREEMPT_RT covers only a limited number of
architectures and it disables some functionality which limits the coverage
further.

Limit the replacement to PREEMPT_RT for now.  This is also done
kmap_atomic().

Link: https://lkml.kernel.org/r/20230310162905.O57Pj7hh@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reported-by: Richard Weinberger <richard.weinberger@gmail.com>
Link: https://lore.kernel.org/CAFLxGvw0WMxaMqYqJ5WgvVSbKHq2D2xcXTOgMCpgq9nDC-MWTQ@mail.gmail.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/io-mapping.h b/include/linux/io-mapping.h
index 09d4f17..7376c1d 100644 (file)
--- a/include/linux/io-mapping.h
+++ b/include/linux/io-mapping.h
@@ -69,7 +69,10 @@ io_mapping_map_atomic_wc(struct io_mapping *mapping,
 
        BUG_ON(offset >= mapping->size);
        phys_addr = mapping->base + offset;
-       preempt_disable();
+       if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+               preempt_disable();
+       else
+               migrate_disable();
        pagefault_disable();
        return __iomap_local_pfn_prot(PHYS_PFN(phys_addr), mapping->prot);
 }
@@ -79,7 +82,10 @@ io_mapping_unmap_atomic(void __iomem *vaddr)
 {
        kunmap_local_indexed((void __force *)vaddr);
        pagefault_enable();
-       preempt_enable();
+       if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+               preempt_enable();
+       else
+               migrate_enable();
 }
 
 static inline void __iomem *
@@ -162,7 +168,10 @@ static inline void __iomem *
 io_mapping_map_atomic_wc(struct io_mapping *mapping,
                         unsigned long offset)
 {
-       preempt_disable();
+       if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+               preempt_disable();
+       else
+               migrate_disable();
        pagefault_disable();
        return io_mapping_map_wc(mapping, offset, PAGE_SIZE);
 }
@@ -172,7 +181,10 @@ io_mapping_unmap_atomic(void __iomem *vaddr)
 {
        io_mapping_unmap(vaddr);
        pagefault_enable();
-       preempt_enable();
+       if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+               preempt_enable();
+       else
+               migrate_enable();
 }
 
 static inline void __iomem *